1. Field of the Invention
This invention relates to an objective lens construction particularly suitable for use in endoscopes.
2. Prior Art
The endoscopes such as bronchoscopes which are inserted into the bronchial tube for a medical examination or for a therapeutic purpose are required to be as thin as possible at least in that portion which is to be inserted into the body. Likewise, the objective lens to be mounted in an observation window at the fore end of the thin insert portion of an endoscope is required to have as small a size as possible and to be of a wide angle. In order to meet these requirements, there has thus far been adopted a lens construction as shown in FIG. 11.
As clear therefrom, the conventional lens construction is composed of three lens elements arranged in three groups, namely, a series of three juxtaposed lens elements consisting of, from the object side, a first lens element L.sub.1a consisting of a plano-concave negative lens having a plane surface disposed on the object side and a concave surface on the image side, a second lens element L.sub.2a consisting of a plano-convex positive lens having a plane surface on the object side and a convex surface on the image side, and a third lens element L.sub.3a consisting of a plano-convex lens having a convex surface on the object side and a plane surface on the image side. A stop S is interposed between the first and second lens elements L.sub.1a and L.sub.2a. Located on the image side of the third lens element L.sub.3a is the incident end of an image guide consisting of a bundle of optical fiber which transfers images of objects under observation or a solid image pick-up element like CCD which picks up images of objects under observation.
Using expressions by the following symbols, m: surface numbers counted successively from the object side;
r.sub.1, r.sub.2, . . . r.sub.6 : radii of curvature of respective lenses;
d.sub.1, d.sub.2, . . . d.sub.5 : lens thicknesses or airspaces;
n.sub.1, n.sub.3, n.sub.5 : refractive indices relative to line d;
.gamma..sub.1, .gamma..sub.3, .gamma..sub.5 : Abbe's numbers of respective lenses;
f: focal length of the total lens system; and
2.omega.: angle of view
the aberration characteristics curves as shown in the diagrams of FIG. 12 are obtained when
______________________________________ m r d n .nu. ______________________________________ 1 .infin. 0.362 1.55920 53.9 2 1.090 0.517 3 .infin. 0.828 1.88300 41.0 4 -0.805 0.310 5 1.150 1.075 1.88300 41.0 6 .infin. f = 0.5 field angle 2.omega. = 90.degree. ______________________________________
As clear from the aberration diagrams, monochromatic light aberrations such as spherical aberration, field curvature and distortion are corrected sufficiently, but lateral chromatic aberration still remains in an undesirably large degree, lowering the resolution of the lens system. This lateral chromatic aberration becomes greater especially in case of the endoscope which employs a wide angle lens system for the purpose of capturing in a wide view angle the images at a close distance from the observation window of the inserted instrument during observation.
In this regard, it is known that the chromatic aberration can be improved by the use of an achromatizing cemented lens as shown at L.sub.3b in FIG. 13. Namely, there has been known a lens construction where an achromatic cemented lens L.sub.3b is located on the image side of a plano-convex lens of the second lens element L.sub.2b, interposing a stop S between the second lens element L.sub.2b and a plano-concave lens of the first lens element L.sub.1b which is located on the object side. This lens construction where
______________________________________ m r d n .nu. ______________________________________ 1 .infin. 0.357 1.81550 44.4 2 0.794 0.469 3 .infin. 0.816 1.77250 49.6 4 -0.729 0.398 5 0.85 0.684 1.60738 56.7 6 -0.85 0.565 1.80518 25.4 7 .infin. ______________________________________ (m, r, d, n and .nu. have the same meaning as defined above) can favorabl correct the various aberrations including the lateral chromatic aberration.
As mentioned hereinbefore, the objective lens for endoscope is required to use a wide angle lens system and at the same time to be as small as possible in size of the lens system as a whole. However, the achromatizing cemented lens L.sub.3b requires strict alignment of the optical axes of two lens elements when joining them together, as a result necessitating to combine a concave lens element with a convex lens element which has a smaller diameter than the concave lens element. Needless to say, it is extremely difficult to polish lens elements of such a small diameter. It is also extremely difficult to discriminate the difference in radius of curvature between the opposite surfaces of a convex lens element to be joined. Therefore, there have to be employed lens elements with the same radius of curvature on the opposite surfaces in order to preclude errors in assembling process. This imposes a great restriction on lens designs.